Optical navigation devices use a light source to illuminate a navigation surface so that an optical imager can generate digital images for computing motion. For example, an optical imager can image a finger on a surface and generate a navigation signal based on comparisons of sequential images. However, as the size of conventional navigation devices is reduced, the physical space for the imager for detecting the optical navigation input are also reduced.
Some conventional optical navigation device packages have a thickness, or optical height, of about 2.5 mm. The optical height refers to the distance from an optical finger interface surface to a corresponding sensor. The optical height is also referred to as the optical track. An optical track, or thickness, of 2.5 mm is considered too thick for some implementations of handheld devices such as cellular telephones and small portable personal computing (PC) peripherals.
Many optical navigation devices use a discrete element called a light guide to channel light from the light source to the navigation surface. Light from the light source travels through the light guide until it intersects with a boundary of the light guide. When the light intersects with the boundary of the light guide, it either partially exits the light guide and partially reflects within the light guide, or completely reflects off the boundary of the light guide, traveling in a new direction within the light guide. This type of reflection is called “total internal reflection” (TIR), and is dependent upon the angle of incidence of the light with the boundary and the refractive indices of the light guide and the material outside the light guide. The light guide is typically a molded piece of clear plastic, and the thickness of the light guide element directly adds to the overall thickness of the optical navigation device.